This invention relates generally to computed tomographic imaging methods and systems, and more particularly to computed tomographic imaging methods and systems for multi-slice imaging of cyclically moving objects.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
In a helical scan, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object while a table supporting the object is moved through the imaging plane. The distance that the table advances during one revolution of the x-ray source and detector is measured by the pitch of the helical scan. A large pitch indicates a large movement of the table per revolution of the x-ray source and detector.
In half scan reconstruction, images are reconstructed from projection data collected during less than a full revolution of the x-ray source and detector around an object. Typically, a xe2x80x9chalf scanxe2x80x9d reconstruction utilizes data obtained from a total view angle of 180xc2x0 plus one fan angle. A xe2x80x9cfan anglexe2x80x9d refers to an angle of the xe2x80x9cfanxe2x80x9d of the x-ray beam that can be detected by the detector in the imaging plane. This can be considered to be equal to an angular extent of the detector in the imaging plane, because in at least one known CT imaging system, the x-ray beam emitted by the x-ray source is as wide or wider in angular coverage than is the detector.
In at least one diagnostic procedure that utilizes a CT imaging system, a patient""s heart is scanned and image so that calcification deposits can be observed and scored. The patient""s heart is cyclically beating during this procedure. To reduce motion-induced artifacts, half scan reconstructions of images that represent the heart at the same phase of the patient""s cardiac cycle are produced. Usually, a relatively quiescent phase, for example, a phase immediately before systole is selected for reconstruction. The entire volume, or at least, a large part of the volume of the heart is imaged in this manner. Therefore, in at least one known variation of this procedure, calcification scoring is performed with a multi-slice CT scanner. A multi-slice CT scanner has more than one row of detectors configured to obtain a plurality of image slices parallel to the xe2x80x9cplanexe2x80x9d of the fan beam. The thickness of the fan beam is such that each row of detectors is able to obtain attenuation measurements representative of essentially parallel slices of the patient""s body.
It is known that high pitch helical cardiac imaging is employed in some calcification scoring procedures. A 3:1 pitch is used, for example, with scanners having detectors configured to acquire four slices at a time. A xe2x80x9c3:1 pitchxe2x80x9d indicates that, as the x-ray source and detector completes one rotation around the patient""s body, the table advances an amount equal to the thickness of three detector slices. These high pitches are used to reduce the amount of time necessary for obtaining a sufficient number of images for accurate scoring estimates.
It has been found that at least one known helical scanning technique misses space between images of adjacent cardiac cycles scanned for cardiac calcification scoring. This missed space can cause an inaccurate calculation of calcification scores. For example, in at least one known imaging system, at a pitch of 3:1, a heart beating less than 75 bpm (beats per minute) cannot be scanned at a speed of 0.8 seconds per gantry revolution. Heartbeats less than 60 bpm cannot be scanned at a speed of 1.0 second per gantry revolution.
A reason that complete coverage is not attainable is that straight segments of valid data spanning an angle xcfx80+xcex3 are not available for all slices representing the heart at corresponding phases of a cardiac cycle. (xcex3 is at least 0xc2x0 and less than or equal to a fan angle.) For example, FIG. 3 represents a prior art four-slice helical scan of two consecutive cardiac cycles. Z-axis positions of isocenters of each detector row 2A, 1A, 1B, and 2B are shown as a function of gantry revolution during two consecutive cardiac cycles. A scan speed of 0.8 seconds per rotation and a heart rate of 60 bpm is represented, so that a cardiac cycle is competed in 1.25 gantry revolutions. Each gantry revolution represents a translation of an x-ray source and detector through an angle 2xcfx80.
Each data segment 50, 52, 54, 56, 58, 60, 62, and 64 is obtained using a linear interpolation of data in an adjacent pair of a set of four detector rows 2A, 1A, 1B, and 2B. The data in each segment is centered in time around a constant (or nearly constant) phase xcfx86 of the patient""s cardiac cycle. Segments 52 and 60 for detector row 1A can be linearly interpolated as complete half scan reconstructions because of the presence of adjacent detector rows 2A and 1B. Similarly, segments 54 and 62 for detector row 1B can also be linearly interpolated as complete half scan reconstructions because of the presence of adjacent detector rows 1A and 2B. However, only data spanning an angle (xcfx80+xcex3)/2 is available for segments 50 and 58 for peripheral slices reconstructed from detector row 2A, because there is only one adjacent detector row 1A. Similarly, only data spanning an angle (xcfx80+xcex3)/2 is available for segments 56 and 64 for peripheral slices reconstructed from detector row 2B, because there is only one adjacent detector row 1B. Half scan reconstruction is available only for central slices from segments 52, 54, 60, and 62. Thus, there is a missed space between images of two cardiac cycles that detector rows 2A and 2B cannot adequately scan.
It would therefore be desirable to provide methods and apparatus for reconstructing high pitch multi-slice helical cardiac imaging that did not suffer from lost spaces between images of two cardiac cycles.
There is therefore provided, in one aspect of the present invention, a method for scanning an object with a multi-slice CT imaging system having multiple detector rows each having an isocenter. The method includes steps of helically scanning an object with the multi-slice CT imaging system to obtain data segments including peripheral data segments, combining data from a first peripheral data segment with an opposite, second peripheral segment to form a data set for reconstruction of an image slice; and reconstructing the combined data into image slices.
The above-described method and corresponding apparatus provides high pitch multi-slice helical cardiac imaging that does not suffer from lost spaces between images of two cardiac cycles. As a result, cardiac calcification scoring is made more accurate. In addition, methods and corresponding apparatus of the present invention are more generally useful in imaging other objects having a cyclic motion.